Dependence of neutral temperatures in the lower thermosphere on geomagnetic activity

The neutral temperature Tn in the Earth's lower thermosphere is important to understand the atmospheric conditions. Tn responds to changes in solar irradiance and geomagnetic activity, meaning the atmospheric density and composition may vary strongly in time and space. Unfortunately, there have been few direct measurements of Tn at altitudes between 150 and 200 km. Recently, a new technique has been used to provide the neutral temperature, using measurements of N2 Lyman-Birge-Hopfield (LBH) band emissions from the High-resolution Ionospheric and Thermospheric Spectrograph (HITS) instrument on the Advanced Research and Global Observation Satellite (ARGOS). By fitting the high resolution N2 LBH band profiles, Tn in the lower thermosphere can be retrieved. In this presentation we examine the latitudinal and temporal variations in the temperatures as observed by HITS for 10 days in 2000 and 2001. The observed temperature changes follow variations in geomagnetic activity. Also, we find a good (poor) match during quiet (active) conditions between Tn from HITS and model calculations using the NRLMSISE-00 code. The agreement found reveals an almost linear relationship with the AL-index, yielding a correlation coefficient of 0.87.